1. Field of the Invention
The invention pertains to the field of acoustic sensor's and more particularly to acoustic sensors responsive to accoustic particle acceleration.
2. Description of the Prior Art
Acoustic sensors in an acoustic sensor array positioned at a source of acoustic noise, which may be a thick vibrating metallic plate, for sensing acoustic signals incident to the region of the noise source are usually isolated from the noise source by an acoustic decoupler or baffle. Generally, such a baffle is a relatively thin layer of material exhibiting a low acoustic impedance, relative to the propagating medium, which covers the entire area in back of the receiving array, e.g. between the array and the noise source, and isolates the array, typically comprising pressure sensitive acoustic sensors, from the acoustic noise emitted from the noise source. Though the low acoustic impedance provides significant attenuation of the radiated noise, its presence adversely affects the signal response of the acoustic sensors in the array.
A pressure wave incident to a low impedance baffle is reflected with an amplitude that is approximately equal to that of the incident wave and a phase that is approximately 1800 from that of the incident wave. The reflected and incident waves add, creating a pressure wave at the acoustic sensor having an amplitude that is significantly lower than that of the incident wave. Since the electrical signal output of the acoustic sensor is a function of the amplitude of the pressure wave at the sensor, the reduced amplitude causes a concomitant reduction in the sensor's electrical signal output and a reduced signal-to-noise ratio from that which would have been provided had the reflection from the baffle not been present. Two solutions to this problem have been implemented. One positions the acoustic sensor array a quarter wavelength from the baffle whereat the reflected wave is in-phase with the incident wave, thus providing a pressure wave amplitude that is greater than that of the incident pressure wave. The second solution interposes a high acoustic impedance between the baffle and the acoustic sensor. Reflections from this high impedance are in-phase with the incident acoustic wave and the resulting pressure wave amplitude at the acoustic sensor is similar to that of the first solution.
Since the acoustic sensors in the first solution are positioned a quarter wavelength from the low impedance baffle, this solution severely limits the frequency bandwidth of the system. Further, the required quarter wavelength standoff requirement increases as the frequency decreases, becoming unacceptable at the low acoustic frequencies.
The second solution requires a massive material interposed between the acoustic noise source and the low impedance baffle to provide the desired high impedance. Typically this massive material is a thick steel plate, the thickness of which, to establish the required impedance level, is inversely proportional to the desired lowest signal frequency in the system spectrum. At the lower acoustic frequencies the steel plate becomes massive and may adversely effect the stability of the noise source, especially if the acoustic sensor array has a large area. Thus, array area and operating frequency, which are inversely proportional for a desired acoustic beamwidth, must be considered in the design of a practical array of pressure sensitive acoustic sensors.
It is therefore an object of this invention to provide an acoustic sensor which does not require a large standoff distance or a massive correction plate to provide an operational acoustic array.